Steam generators of nuclear reactors cooled by pressurized water are components of large dimensions comprising an enclosure of general cylindrical shape, placed with its axis vertical, in a lateral arrangement with respect to the vessel of the nuclear reactor enclosing the core, within a stationary concrete structure comprising vertical walls, called a bunker.
Each of the steam generators is placed on a loop of the primary circuit of the nuclear reactor constituted by pipelines of large diameter connecting the vessel to the primary part of the steam generator. One of these pipelines, called hot leg, ensures a direct connection between the vessel and the inlet part to the water box of the steam generator ensuring the distribution of pressurized water for cooling the reactor in the tubes of the bundle of the steam generator, which tubes are contained within the enclosure.
A second pipeline or intermediate leg connects the outlet part of the water box of the steam generator to a primary pump ensuring the circulation of the cooling water in the loop of the primary circuit.
The primary pump is connected to the vessel of the steam generator, in such a manner as to return the cooled cooling water into the steam generator within the vessel, via a third pipeline, called a cold leg.
The steam generator rests, within the concrete bunker, on vertically articulated support columns connected at their lower part to support plates embedded in a concrete slab constituting the bottom of the bunker.
Moreover, the steam generator is held within the bunker by an assembly of tie rods and dampers interposed between the vertical walls of the bunker and a ring for holding the wrapper of the steam generator situated in the vicinity of the middle part of the steam generator, between its lower end constituting the wall of the water box and its upper end from where the steam produced within the generator exits.
The steam generator also comprises transverse holding devices or supports ensuring the guiding of the steam generator and holding it in the event of an accident. These devices or supports are arranged in the vicinity of the primary bottom of the steam generator constituting the wall of the water box.
The transverse holding assembly of the steam generator generally comprises two lateral supports placed in symmetric positions with respect to the vertical plane containing the axis of the hot leg and a front support of which the contact zone with the steam generator is situated in the vicinity of the vertical plane containing the axis of the hot leg and in an opposite position with respect to the hot leg connected to the water box.
Holding supports for the Steam generator comprise a bearing element intended to come into contact with or into the vicinity of a bearing surface of the steam generator which can be constituted either by the primary bottom of the generator, i.e. the external surface of the water box, or by the outer lateral surface of the tube plate upon which the bundle of tubes for the steam generator is fixed and which provides the junction between the water box and the wrapper of the steam generator, or alternatively by a welded ring surrounding the lower part of the steam generator.
The holding devices each comprise a base plate anchored in the concrete of one of the walls of the bunker to which the bearing element of the support is connected, which can be constituted by a second base plate fixed on the first in a parallel arrangement with a certain spacing.
Generally, an energy absorbing system is interposed between the two base plates, in such a manner as to damp out impacts or energy connected with movement of the steam generator in the event of an accident.
These energy absorbers can be constituted by "cushions" made of stainless steel wires.
In the case of the front support, the energy absorption system is generally constituted by several rows of steel bars arranged parallel to each other in each of the rows and in arrangements perpendicular from one row to the next and staggered, in such a manner as to absorb the energy by deformation or bending, in the event of an accidental movement of the steam generator in the direction of the corresponding wall of the bunker.
When the steam generator is installed in the bunker, the supports which ensure transverse holding of its lower part must be adjusted, in such a manner that a perfectly defined play remains between the bearing element of each of the supports and the corresponding bearing surface of the steam generator.
The play is defined as a function of the foreseeable movements of the steam generator, in normal operation and during an accident due to a cause internal to the nuclear reactor, for example due to rupture of a primary pipe or to steam or due to a cause which is totally external to the nuclear reactor, such as an earthquake.
The play can be variable as a function of the supports, in that the movements of the steam generator are not identical in all directions.
The bearing element of the front support is thus generally arranged at a significant distance, of the order of 50 mm from the surface of the steam generator, the movements of the steam generator in the axis of the hot leg having a relatively significant amplitude. Due to this significant play, the front supports do not come into contact with the steam generator and do not participate in holding the steam generator in the event of an earthquake.
The lateral supports arranged on the side of the plane of symmetry of the hot leg on which are situated the intermediate leg and the primary motor-driven pump group must be arranged with zero play with respect to the bearing surface of the steam generator. Thus, the guiding of the steam generator with respect to this reference is assured, both during normal operation of the steam generator and in the event of an accident.
The other lateral supports situated in a symmetric position with respect to an axial plane of the hot leg comprise bearing elements arranged with a certain play with respect to the surface of the steam generator.
The adjustment of the supports constituting the shimming operation of the steam generator must be carried out during the mounting of any new generator in a nuclear power station, whether it concerns a generator intended for a new power station before it is put into service or a replacement steam generator intended for a nuclear reactor which has operated for a longer or shorter period of time.
The adjustment of the play of the various supports is carried out by fixing shims of appropriate thickness on an external base plate of the support directed towards the steam generator and connected to the fixed base plate anchored in the concrete of the wall of the bunker. The external base plate and the shims in this case constitute the bearing element of the support intended to come into contact with the bearing surface of the steam generator.
The shimming of the steam generator is carried out in several phases.
During hot testing of the nuclear reactor, the primary fluid being at 155 bar and 286.degree. C. during these tests, which corresponds to the nominal operational conditions of the reactor, the lower lateral and front supports of the steam generator are installed and preadjusted, and the play between the steam generator and the external base plates of the supports which do not comprise any shims in this phase is measured.
The measured values of the play during the hot testing of the steam generator are communicated to the research department of the constructor of the nuclear reactor.
The necessary shim thicknesses are calculated as a function of the play measured and the movements of thermal origin of the steam generator which are calculated.
Prefabricated shims are labelled as a function of their destination and equipped with lifting means to enable them to be handled.
The shims are then pierced, machined to their final dimensions, checked and delivered to the site of the nuclear reactor. The shims which are identified on site are installed and fixed either by screws, or by welding, in the bunker of the steam generator, the primary circuit being at a low temperature.
It is necessary to carry out the operations described above on an assembly of about thirty shims, in order to provide the shimming of a steam generator, each of the lateral supports and the front support requiring the presence of ten shims to ensure its adjustment.
These operations are thus long and complex, which has drawbacks, especially in replacing a worn steam generator. In fact, in this case, it is necessary to reduce to a minimum the amount of time needed to replace the steam generator and the total shut-down time of the nuclear reactor.